Iron-based sintered body and method of manufacturing the same

ABSTRACT

Provided is an iron-based sintered body having excellent mechanical properties. In the sintered body, the area fraction of pores is 15% or less and the area-based median size D50 of the pores is 20 82 m or less.

TECHNICAL FIELD

This disclosure relates to an iron-based sintered body, and relates inparticular to an iron-based sintered body suitable for manufacturinghigh strength sintered parts for automobiles, the sintered body havinghigh sintered density and having reliably improved tensile strength andtoughness (impact energy value) after performing the processes ofcarburizing, quenching, and tempering on the sintered body. Further,this disclosure relates to a method of manufacturing the iron-basedsintered body.

Powder metallurgical techniques enable producing parts with complicatedshapes in shapes that are extremely close to product shapes (so-callednear net shapes) with high dimensional accuracy, and consequentlysignificantly reducing machining costs. For this reason, powdermetallurgical products are used for various machines and parts in manyfields.

In recent years, there is a strong demand for powder metallurgicalproducts to have improved toughness in terms of improving the strengthfor miniaturizing parts and reducing the weight thereof and safety. Inparticular, for powder metallurgical products (iron-based sinteredbodies) which are very often used for gears and the like, in addition tohigher strength and higher toughness, there is also a strong demand forhigher hardness in terms of wear resistance. In order to meet theabove-mentioned demands, iron-based sintered bodies of which components,structures, density and the like are controlled suitably are required tobe developed, since the strength and toughness of an iron-based sinteredbody varies widely depending on those properties.

Typically, a green compact before being subjected to sintering isproduced by mixing iron-based powder with alloying powders such ascopper powder and graphite powder and a lubricant such as stearic acidor lithium stearate to obtain mixed powder; filling a mold with themixed powder; and compacting the powder.

The density of a green compact obtained through a typical powdermetallurgical process is usually around 6.6 Mg/m³ to 7.1 Mg/m³. Thegreen compact is then sintered to form a sintered body which in turn isfurther subjected to optional sizing or cutting work, thereby obtaininga powder metallurgical product. Further, when even higher strength isrequired, carburizing heat treatment or bright heat treatment may beperformed after sintering.

Based on the components, iron-based powders used here are categorizedinto iron powder (e.g. iron-based powder and the like) and alloy steelpowder. Further, when categorized by production method, iron-basedpowders are categorized into atomized iron powder and reduced ironpowder. Within these categories specified by production methods, theterm “iron powder” is used with a broad meaning encompassing alloy steelpowder as well as iron-based powder.

In terms of obtaining a sintered body with high strength and hightoughness, it is advantageous that iron-based powder being a maincomponent in particular allows alloying of the powder to be promoted andhigh compressibility of the powder to be maintained.

First, known iron-based powders obtained by alloying include:

-   (1) mixed powder obtained by adding alloying element powders to    iron-based powder,-   (2) pre-alloyed steel powder obtained by completely alloying    alloying elements,-   (3) partially diffusion alloyed steel powder (also referred to as    composite alloy steel powder) obtained by partially adding alloying    element powders in a diffused manner to the surface of particles of    iron-based powder or pre-alloyed steel powder.

The mixed powder (1) mentioned above advantageously has highcompressibility equivalent to that of pure iron powder. However, insintering, the alloying elements are not sufficiently diffused in Fe andform a non-uniform microstructure, which would result in poor strengthof the resulting sintered body. Further, since Mn, Cr, V, Si, and thelike are more easily oxidized than Fe, when these elements are used asthe alloying elements, they get oxidized in sintering, which wouldreduce the strength of the resulting sintered body. In order to suppressthe oxidation and reduce the amount of oxygen in the sintered body, itis necessary that the atmosphere for sintering, and the CO₂concentration and the dew point in the carburizing atmosphere arestrictly controlled in the case of performing carburizing aftersintering. Accordingly, the mixed powder (1) mentioned above cannot meetthe demands for higher strength in recent years and has become unused.

On the other hand, when the pre-alloyed steel powder obtained bycompletely alloying the elements of (2) mentioned above is used, thealloying elements can be completely prevented from being segregated, sothat the microstructure of the sintered body is made uniform, leading tostable mechanical properties. In addition, also in the case where Mn,Cr, V, Si, and the like are used as the alloying elements, the amount ofoxygen in the sintered body can be advantageously reduced by limitingthe kind and the amount of the alloying elements. However, when thepre-alloyed steel powder is produced by atomization from molten steel,oxidation in the atomization of the molten steel and solid solutionhardening of steel powder due to complete alloying would be caused,which makes it difficult to increase the density of the green compactafter compaction (forming by pressing). When the density of the greencompact is low, the toughness of the sintered body obtained by sinteringthe green compact is low. Therefore, also when the pre-alloyed steelpowder is used, demands for higher strength and higher toughness cannotbe met.

The partially diffusion alloyed steel powder (3) mentioned above isproduced by adding alloying elements to iron-based powder or pre-alloyedsteel powder, followed by heating under a non-oxidizing or reducingatmosphere, thereby partially diffusion bonding the alloying elementpowders to the surface of particles of iron-based powder or pre-alloyedsteel powder. Accordingly, advantages of the iron-based mixed powder of(1) above and the pre-alloyed steel powder of (2) above can be obtained.

Thus, when the partially diffusion pre-alloyed steel powder is used,oxygen in the sintered body can be reduced and the green compact canhave a high compressibility equivalent to the case of using pure ironpowder. Therefore, the sintered body has a multi-phase structureconsisting of a completely alloyed phase and a partially concentratedphase, increasing the strength of the sintered body.

As basic alloy components used in the partially diffusion alloyed steelpowder, Ni and Mo are used heavily.

Ni has the effect of improving the toughness of a sintered body. AddingNi stabilizes austenite, which allows more austenite to remain asretained austenite without transforming to martensite after quenching.Further, Ni serves to strengthen the matrix of a sintered body by solidsolution strengthening.

Meanwhile, Mo has the effect of improving hardenability. Accordingly, Mosuppresses the formation of ferrite during quenching, allowing bainiteor martensite to be easily formed, thereby strengthening the matrix ofthe sintered body. Further, Mo is contained as a solid solution in amatrix to solid solution strengthen the matrix, and forms fine carbidesto strengthen the matrix by precipitation.

As an example of the mixed powder for high strength sintered parts usingthe above-described partially diffusion alloyed steel powder, JP 3663929B2 (PTL 1) discloses mixed powder for high strength sintered partsobtained by mixing Ni: 1 mass % to 5 mass %, Cu: 0.5 mass % to 4 mass %,and graphite powder: 0.2 mass % to 0.9 mass % to alloy steel powder inwhich Ni: 0.5 mass % to 4 mass % and Mo: 0.5 mass % to 5 mass % arepartially alloyed. The sintered material described in PTL 1 contains 1.5mass % of Ni at minimum, and substantially contains 3 mass % or more ofNi according to Examples of PTL 1. This means that a large amount of Nias much as 3 mass % or more is required to obtain a sintered body havinga high strength of 800 MPa or more. Further, obtaining a material havinga high strength of 1000 MPa or more by subjecting a sintered body tocarburizing, quenching, and tempering also requires a large amount of Nias much as for example 3 mass % or 4 mass %.

However, Ni is an element which is disadvantageous in terms ofaddressing recent environmental problems and recycling, so its use isdesirably avoided as possible. Also in respect of cost, adding severalmass % of Ni is significantly disadvantageous. Further, when Ni is usedas an alloying element, sintering is required to be performed for a longtime in order to sufficiently diffuse Ni in iron powder or steel powder.Moreover, when Ni being an austenite phase stabilizing element is notsufficiently diffused, a high Ni concentration area is stabilized as theaustenite phase (hereinafter also referred to as y phase) and the otherarea where Ni is hardly contained is stabilized as other phases,resulting in a non-uniform metal structure in the sintered body.

As a Ni-free technique, JP 3651420 B2 (PTL 2) discloses a techniqueassociated with partially diffusion alloyed steel powder of Mo free ofNi. That is, PTL 2 states that optimization of the Mo content results ina sintered body having high ductility and high toughness that can resistrepressing after sintering.

Further, regarding a high density sintered body free of Ni, JPH04-285141 A (PTL 3) discloses mixing iron-based powder having a meanparticle diameter of 1 μm to 18 μm with copper powder having a meanparticle diameter of 1 μm to 18 μm at a weight ratio of 100:(0.2 to 5),and compacting the mixed powder and sintering the green compact. In thetechnique disclosed in PTL 3, iron-based powder having a mean particlediameter that is extremely smaller than that of typical one is used, sothat a sintered body having a density as extremely high as 7.42 g/cm³ ormore can be obtained.

WO 2015/045273 A1 (PTL 4) discloses that a sintered body having highstrength and high toughness is obtained using powder free of Ni, inwhich Mo is adhered to the surface of iron-based powder particles bydiffusion bonding to achieve a specific surface area of 0.1 m²/g ormore.

Further, JP 2015-014048 A (PTL 5) discloses that a sintered body havinghigh strength and high toughness is obtained using powder in which Mo isadhered to iron-based powder particles containing reduced iron powder bydiffusion bonding.

JP 2015-004098 A (PTL 6) describes that Fe-Mn-Si powder is added to ironpowder particles of a small particle size and the mixed powder is warmcompacted in a lubricated mold, thereby reducing the maximum pore lengthof the sintered body to obtain a sintered body having high strength andhigh toughness.

CITATION LIST Patent Literature

PTL 1: JP 3663929 B2

PTL 2: JP 3651420 B2

PTL 3: JP H04-285141 A

PTL 4: WO 2015/045273 A1

PTL 5: JP 2015-014048 A

PTL 6: JP 2015-004098 A

SUMMARY Technical Problem

However, the sintered materials obtained in accordance with thedescription of PTL 2, PTL 3, PTL 4, PTL 5, and PTL 6 above have beenfound to have the following respective problems.

The technique disclosed in PTL 2 is designed to achieve high strength byrecompression after sintering. Accordingly, when a sintered material ismanufactured by a typical metallurgical process, both sufficientstrength and toughness are hardly achieved at the same time.

Further, the iron-based powder used for the sintered material describedin PTL 3 has a mean particle diameter of 1 μm to 18 μm which is smallerthan normal. Such a small particle diameter results in poor flowabilityof the mixed powder inducing cracking and chipping of the green compactdue to unevenness of the powder in filling the mold. Therefore, it isdifficult to obtain a sintered body having sufficient strength andtoughness.

Further, since the powder described in PTL 4 has extremely largespecific surface area, use of such powder results in low flowability ofthe powder and induces cracking and chipping of the green compact due tounevenness of the powder in filling the mold. Therefore, it is difficultto obtain a sintered body having sufficient strength and toughness.

Also for the sintered body described in PTL 5, as with the techniquedescribed in PTL 4, reduced iron powder having extremely large specificsurface area is used, which results in low flowability of the powder andinduces cracking and chipping of the green compact due to unevenness ofthe powder in filling the mold. Therefore, it is difficult to obtain asintered body having sufficient strength and toughness.

The toughness of the sintered body disclosed in PTL 6 is increasedmainly by limiting the maximum pore length; however, high strength andtoughness are hardly achieved by only limiting the maximum pore length,and further improvement is required.

It could be helpful to provide an iron-based sintered body havingexcellent mechanical properties as well as a method of manufacturing thesame.

Solution to Problem

With a view to achieve the above objective, we made various studies toobtain a sintered body having both high strength and high toughness. Asa result, we discovered the following:

-   -   for an iron-based sintered body obtained by pressing mixed        powder made of iron-based powder and additives and then        sintering, adjusting the mean diameter of pores in the sintered        body contributes to the improvement in the impact energy value        due to the dispersion of stress concentrations in the structure.

This disclosure is based on the aforementioned discoveries and furtherstudies. Specifically, the primary features of this disclosure aredescribed below.

1. An iron-based sintered body, comprising an area fraction of pores inthe iron-based sintered body of 15% or less, and an area-based mediansize D50 of the pores of 20 μm or less.

2. The iron-based sintered body according to 1. above, comprising Mo,Cu, and C.

3. The iron-based sintered body according to 2. above, comprising Mo inan amount of 0.2 mass % to 1.5 mass %, Cu in an amount of 0.5 mass % to4.0 mass %, and C in an amount of 0.1 mass % to 1.0 mass %.

4. The iron-based sintered body according to any one of 1. to 3. above,wherein the iron-based sintered body has been carburized, quenched, andtempered.

5. A method of manufacturing an iron-based sintered body, the methodcomprising: compacting (i) partially diffusion alloyed steel powder inwhich Mo is adhered to the surface of particles of iron-based powder bydiffusion bonding with (ii) mixed powder for powder metallurgy obtainedby mixing at least Cu powder and graphite powder at a pressure of 400MPa or more to obtain a compact; and then sintering the obtained compactat 1000° C. or higher for 10 min or more.

6. The method of manufacturing a high strength according to 5. above,the method further comprising carburizing, quenching, and temperingafter sintering the obtained compact.

7. The method of manufacturing an iron-based sintered body, according to5. or 6. above, wherein the mixed powder for powder metallurgy containsMo in an amount of 0.2 mass % to 1.5 mass % and the balance consistingof Fe and incidental impurities.

8. The method of manufacturing an iron-based sintered body, according toany one of 5. to 7. above, wherein the partially diffusion alloyed steelpowder has a mean particle diameter of 30 μm to 120 μm and a specificsurface area of less than 0.10 m²/g, and a circularity of particles ofthe partially diffusion alloyed steel powder that have a diameter in arange of 50 μm to 100 μm is 0.65 or less.

9. The method of manufacturing an iron-based sintered body, according toany one of 5. to 8. above, wherein the amount of the Cu powder mixed is0.5 mass % to 4.0 mass % of the mixed powder for powder metallurgy.

Advantageous Effect

This disclosure can provide an iron-based sintered body having both highstrength and high toughness.

DETAILED DESCRIPTION

Our methods and products will be described in detail below.

The area fraction of pores in the disclosed sintered body is 15% or lessand the area-based median size D50 of the pores is 20 μm or less.

Pores are unavoidably formed in the iron-sintered body obtained bysintering a green compact obtained by compacting alloy steel powder forpowder metallurgy, and it is important to control the pores forimproving the strength and toughness of the sintered body. That is,since smaller pores hardly act as starting points of cracks, it isimportant that the area-based median size D50 of the pores is 20 μm orless. More preferably, the area-based median size D50 is 15 μm or less.When the median size D50 exceeds 20 μm, the toughness is significantlyreduced.

Here, the median size D50 of the pores can be measured in the followingmanner.

First, a sintered body is embedded in a thermosetting resin. A crosssection is then mirror-polished and the cross section is imaged using anoptical microscope at 100× magnification over a field of view of 843 μm×629 μm. The cross-sectional area A of all the pores in 20 fieldsrandomly selected from the resulting micrograph of the cross section ismeasured. The equivalent circle diameter d_(c) that is the diameter of acircle having an area equal to the measured cross-sectional area isdetermined in accordance with the following equation (I). Next, theareas of the pores are integrated in ascending order of the circleequivalent diameter and a circle equivalent diameter at which theintegrated value is 50% of the total area of the pores is defined as anarea-based median size D50.

d _(c)=2√{square root over (A/π)}  (I)

As described above, the median size D50 of the pores of the sinteredbody is controlled to 20 μm or less, since a median size D50 exceeding20 μm increases pores having an indefinite shape and such pores becomestress concentrations when deformation occurs, which reduces strengthand toughness.

Here, in order to control the area fraction of the pores in the sinteredbody to 15% or less and the median size D50 of the pores to 20 μm orless, partially diffusion alloyed steel powder of mixed powder forpowder metallurgy which is a material of the sintered body is used. Thepartially diffusion alloyed steel powder is obtained by adhering Mopowder particles to the surface of iron-based powder particles, thesteel powder particles having a mean particle diameter of 30 μm to 120μm a and specific surface area of less than 0.10 m²/g, and particles ofthe steel powder that have a diameter in a range of 50 μm to 100 μm hasa circularity of 0.65. Thus, sintering is promoted in manufacturing asintered body to be described, so that a desired sintered body can beobtained.

Since the number of pores is preferably smaller, the area fraction ofthe pores in the sintered body is controlled to 15% or less. This isbecause since an area fraction of the pores exceeding 15% reduces thecontent of metal in the sintered body, even if the pore diameter isreduced, sufficient strength and toughness cannot be obtained. Note thatmaking the pores in the sintered body be 0% requires significant effortand is not realistic. The pores in the sintered body obtained by thefollowing method is at least approximately 5%.

Here, the area fraction of the pores in the sintered body can becalculated by the following method.

In a manner similar to the above, the cross-sectional area A of all thepores in 20 fields is measured and summed to find the total pore areaA_(t) of all the observed fields. Dividing A_(t) by the total of theareas of all the observed fields gives the area fraction of the pores.

Further, the length of the pores in the sintered body is preferablysmaller. The “mean maximum pore length” that is an indicator of thelength of the pores is calculated as follows. First, the maximum valueof the distance between two points on the circumferential edge of eachpore in the field of the above micrograph of the cross section is foundby image analysis and is defined as the “pore length” of the pore. The“maximum pore length” is longest among the “pore lengths” of all thepores included in a field of view of the micrograph of the crosssection. Further, the “mean maximum pore length” is the arithmetic meanvalue of the maximum pore lengths found for 20 fields selected randomly.Note that in order to achieve sufficient mechanical properties, the meanmaximum pore length is preferably less than 100 μm.

Further, the above sintered body preferably contains Mo, Cu, and C. Mohas the effect of improving hardenability. Cu has the effect ofimproving solid solution strengthening and hardenability of iron-basedpowder. C has the effect of enhancing the strength of iron-basedsintered body by being precipitated as a solid solution or fine carbidein iron. Preferred content range of the respective elements contained inthe disclosed iron-based sintered body is Mo: 0.2 mass % to 1.5 mass %,Cu: 0.5 mass % to 4.0 mass %, and C: 0.1 mass % to 1.0 mass %. When theelements are less than the above range, the strength cannot sufficientlybe increased, whereas when they are added to be more than the aboverange, the structure is extremely hardened and the toughness is reduced.

Next, a method of obtaining the above sintered body will be described.The following method is a mere example, and the disclosed iron-basedsintered body may be obtained by a method other than the followingmethod.

In manufacturing a sintered body by sintering a green compact obtainedby compacting mixed powder for powder metallurgy, the mixed powder ismade into the green compact by compaction using a punch by a techniquein which the compaction is performed while rotating the punch about animaginary axis in the pressing direction. This method can produce moreshear strains in the mixed powder than in typical compaction,facilitating plastic deformation of the mixed powder, and the pores inthe sintered body can have a finer diameter.

Next, a method of manufacturing a sintered body, particularly suitablefor manufacturing a sintered body containing Mo, Cu, and C will bedescribed.

In this method, mixed powder for powder metallurgy containing iron-basedpowder and additives is compacted by a conventional method to form agreen compact, and the green compact is then sintered by a conventionalmethod, thereby obtaining an iron-based sintered body. On this occasion,with a view to increasing the density of the sintered body, it ispreferable that Mo-concentrated portions are formed in sintered neckparts between particles of the iron-based powder in the green compact;iron-based powder having particles with low circularity is used toachieve stronger entanglement between particles of the powder duringcompaction thereby promoting sintering; and the sintering is alsopromoted with suppressed Cu growth. When the density of a sintered bodyis high, both strength and toughness are improved; however, since asintered body obtained by this manufacturing method has a uniform metalstructure, the mechanical properties of the sintered body are stablewith little variation, unlike conventional sintered bodies, for example,those using Ni.

In order to obtain such a sintered body, the sintered body is preferablymanufactured using partially diffusion alloyed steel powder describedbelow as the iron-based powder of the above mixed powder for powdermetallurgy.

Mixed powder for powder metallurgy preferably used in this disclosure isobtained by mixing partially diffusion alloyed steel powder in which Mois adhered by diffusion bonding to the surface of particles ofiron-based powder of which mean particle diameter, circularity, andspecific surface area are appropriate (hereinafter also referred to aspartially alloyed steel powder) with an appropriate amount of Cu powderhaving a mean particle diameter in a range described below as well asgraphite powder.

Mixed powder for powder metallurgy according to this disclosure will nowbe described in detail. Note that “%” herein means “mass %” unlessotherwise specified. Accordingly, the Mo content, the Cu content, andthe graphite powder content each represents the proportion of theelement in the entire mixed powder for powder metallurgy (100 mass %).

(Iron-Based Powder)

As described above, the partially diffusion alloyed steel powder isobtained by adhering Mo to the surface of particles of the iron-basedpowder, and it is preferred that the mean particle diameter is 30 μm to120 μm, the specific surface area is less than 0.10 m²/g, and particleshaving a diameter in a range of 50 μm to 100 μm have a circularity(cross-sectional circularity) of 0.65 or less. Here, when the iron-basedpowder is partially alloyed, the particle diameter and the circularityhardly change. Accordingly, iron-based powder having a mean particlediameter and a circularity in the same range as that of the partiallydiffusion alloyed steel powder is used.

First, the iron-based powder preferably has a mean particle diameter of30 μm to 120 μm and particles having a diameter in a range of 50 μm to100 μm preferably have a circularity (roundness of the cross section) of0.65 or less. For the reasons described below, the partially alloyedsteel powder is required to have a mean particle diameter of 30 μm to120 μm and particles having a diameter in a range of 50 μm to 100 μm arerequired to have a circularity of 0.65 or less. Accordingly, theiron-based powder is also required to meet those conditions.

Here, the mean particle diameter of the iron-based powder and thepartially alloyed steel powder refers to the median size D50 determinedfrom the cumulative weight distribution, and is a particle diameterfound by determining the particle size distribution using a sieveaccording to JIS Z 8801-1, producing the integrated particle sizedistribution from the resulting particle size distribution, and findingthe particle diameter obtained when the oversized particles and theundersized particles constitute 50% by weight each.

Further, the circularity of the particles of iron-based powder andpartially alloyed steel powder can be determined as follows. Although acase of iron-based powder is explained by way of example, thecircularity of partially alloyed steel powder particles is alsodetermined through the same process.

First, iron-based powder is embedded in a thermosetting resin. On thisoccasion, the iron-based powder is embedded to be uniformly distributedin an area with a thickness of 0.5 mm or more in the thermosetting resinso that a sufficient number of cross sections of the iron-based powderparticles can be observed in an observation surface exposed by polishingthe powder-embedded resin. After that, the resin is polished to expose across section of the iron-based powder particles; the cross section ofthe resin is mirror polished; and the cross section is magnified andimaged by an optical microscope. The cross sectional area A and theperipheral length Lp of the iron-based powder particles in the resultingmicrograph of the cross section are determined by image analysis.Examples of software capable of such image analysis include ImageJ (opensource, National Institutes of Health). The circle equivalent diameterdc is calculated from the determined cross-sectional area A. Here, dc iscalculated by the same equation (I) as in the case of the pores.

d _(c)=2√{square root over (A/π)}  (I)

Next, the peripheral length of a circular approximation of each powderparticle Lc is calculated by multiplying the particle diameter dc by thenumber π. The circularity C is calculated from the determined Lc and theperipheral length Lp of the cross section of each iron-based powderparticle. Here, the circularity C is a value defined by the followingequation (II).

When the circularity C is 1, the cross-sectional shape of the particleis a perfect circle, and a smaller C value results in a more indefiniteshape.

C=L _(c)/L _(p)  (II)

Note that iron-based powder means powder having an Fe content of 50% ormore. Examples of iron-based powder include as-atomized powder (atomizediron powder as atomized), atomized iron powder (obtained by reducingas-atomized powder in a reducing atmosphere), and reduced iron powder.In particular, iron-based powder used in this disclosure is preferablyas-atomized powder or atomized iron powder. This is because sincereduced iron powder contains many pores in the particles, sufficientdensity would not be obtained during compaction. Further, reduced ironpowder contains more inclusions acting as starting points of fracture inthe particles than atomized iron powder, which would reduce the fatiguestrength which is one of the important mechanical properties of asintered body.

Specifically, iron-based powder preferably used in this disclosure isany one of as-atomized powder obtained by atomizing molten steel, dryingthe atomized molten steel, and classifying the resulting powder withoutperforming heat treatment for e.g., deoxidation (reduction) anddecarbonization; and atomized iron powder obtained by reducingas-atomized powder in a reducing atmosphere.

Iron-based powder satisfying the above-described circularity can beobtained by appropriately adjusting the spraying conditions foratomization and conditions for additional processes performed after thespraying. Further, iron-based powder having particles of differentcircularities may be mixed and the circularity of the particles of theiron-based powder that have a particle diameter in a range of 50 μm to100 μm may be controlled to fall within the above-described range.

(Partially Diffusion Alloyed Steel Powder)

Partially diffusion alloyed steel powder is obtained by adhering Mo tothe surface of particles of the above iron-based powder, and it isrequired that the mean particle diameter is 30 μm to 120 μm, thespecific surface area is less than 0.10 m²/g, and particles having adiameter in a range of 50 μm to 100 μm have a circularity of 0.65 orless.

Thus, the partially diffusion alloyed steel powder is produced byadhering Mo to the above iron-based powder by diffusion bonding. The Mocontent is set to be 0.2% to 1.5% of the entire mixed powder for powdermetallurgy (100%). When the Mo content is less than 0.2%, thehardenability and strength of a sintered body manufactured using themixed powder for powder metallurgy are poorly improved. On the otherhand, when the Mo content exceeds 1.5%, the effect of improvinghardenability reaches a plateau, and the structure of the sintered bodybecomes rather non-uniform. Accordingly, high strength and toughnesscannot be obtained. Therefore, the content of Mo adhered by diffusionbonding is set to be 0.2% to 1.5%. The Mo content is preferably 0.3% to1.0%, more preferably 0.4% to 0.8 %.

Here, Mo-containing powder can be given as an example of a Mo source.Examples of the Mo-containing powder include pure metal powder of Mo,oxidized Mo powder, and Mo alloy powders such as Fe-Mo (ferromolybdenum)powder. Further, Mo compounds such as Mo carbides, Mo sulfides, and Monitrides can be used as preferred Mo-containing powders. Theses materialpowders can be used alone; alternatively, some of these material powderscan be used in a mixed form.

Specifically, the above-described iron-based powder and theMo-containing powder are mixed in the proportions described above (theMo content is 0.2% to 1.5% of the entire mixed powder for powdermetallurgy (100%)). The mixing method is not particularly limited, andthe powders can be mixed by a conventional method using a Henschelmixer, a cone blender, or the like.

Next, mixed powder of the above-described iron-based powder and theMo-containing powder is heated so that Mo is diffused in the iron-basedpowder through the contact surface between the iron-based powder and theMo-containing powder, thereby joining Mo to the iron-based powder.Partially alloyed steel powder containing Mo can be obtained by thisheat treatment.

As the atmosphere for diffusion-bonding heat treatment, a reducingatmosphere or a hydrogen-containing atmosphere is preferable, and ahydrogen-containing atmosphere is particularly suitable. Alternatively,the heat treatment may be performed under vacuum.

Further, for example when a Mo compound such as oxidized Mo powder isused as the Mo-containing powder, the temperature of the heat treatmentis preferably set to be in a range of 800° C. to 1100° C. When thetemperature of the heat treatment is lower than 800° C., the Mo compoundis insufficiently decomposed and Mo is not diffused into the iron-basedpowder, so that Mo hardly adheres to the iron-based powder. When theheat treatment temperature exceeds 1100° C., sintering betweeniron-based powder particles is promoted during the heat treatment, andthe circularity of the iron-based powder particles exceeds thepredetermined range. On the other hand, when a metal and an alloy, forexample, Mo pure metal and an alloy such as Fe-Mo are used for theMo-containing powder, a preferred heat treatment temperature is in arange of 600° C. to 1100° C. When the temperature of the heat treatmentis lower than 600° C., Mo is not sufficiently diffused into theiron-based powder, so that Mo hardly adheres to the iron-based powder.On the other hand, when the heat treatment temperature exceeds 1100° C.,sintering between iron-based powder particles is promoted during theheat treatment, and the circularity of the partially alloyed steelpowder exceeds the predetermined range.

When heat treatment, that is, diffusion bonding is performed asdescribed above, since partially alloyed steel powder particles areusually sintered together and solidified, grinding and classificationare performed to obtain particles having a predetermined particlediameter described below. Specifically, in order to achieve thepredetermined particle diameter, the grinding conditions are tightenedor coarse powder is removed by classification using a sieve withopenings of a predetermined size, as necessary. In addition, annealingmay optionally be performed.

Specifically, it is important that the mean particle diameter of thepartially alloyed steel powder is in a range of 30 μm to 120 μm. Thelower limit of the mean particle diameter is preferably 40 μm, morepreferably 50 μm. Meanwhile, the upper limit of the mean particlediameter is preferably 100 μm, more preferably 80 μm.

As described above, the mean particle diameter of the partially alloyedsteel powder refers to the median size D50 determined from thecumulative weight distribution, and is a particle diameter found bydetermining the particle size distribution using a sieve according toJIS Z 8801-1, producing the integrated particle size distribution fromthe resulting particle size distribution, and finding the particlediameter obtained when the oversized particles and the undersizedparticles constitute 50% by weight each.

Here when the mean particle diameter of the partially alloyed steelpowder particles is smaller than 30 μm, the flowability of the partiallyalloyed steel powder is reduced, and for example the productivity incompaction using a mold is affected. On the other hand, when the meanparticle diameter of the partially alloyed steel powder particlesexceeds 120 μm, the driving force is weakened during sintering andcoarse pores are formed around the coarse iron-based powder particles.This reduces the sintered density and leads to reduction in the strengthand toughness of a sintered body and the sintered body having beencarburized, quenched, and tempered. The maximum particle diameter of thepartially alloyed steel powder particles is preferably 180 μm or less.

Further, in terms of compressibility, the specific surface area of thepartially alloyed steel powder particles is set to be less than 0.10m²/g. Here, the specific surface area of the partially alloyed steelpowder refers to the specific surface area of particles of the partiallyalloyed steel powder except for additives (Cu powder, graphite powder,lubricant).

When the specific surface area of the partially alloyed steel powderexceeds 0.10 m²/g, the flowability of the mixed powder for powdermetallurgy is reduced. Note that the lower limit of the specific surfacearea is not specified; however, the lower limit of the specific surfacearea achieved industrially is approximately 0.010 m²/g. The specificsurface area can be controlled as desired by adjusting the particle sizeof coarse particles of more than 100 μm and fine particles of less than50 μm after diffusion bonding by sieving. Specifically, the specificsurface area is reduced by reducing the proportion of fine particles orincreasing the proportion of coarse particles.

Further, particles of the partially alloyed steel powder that have adiameter of 50 μm to 100 μm are required to have a circularity of 0.65.The circularity is preferably 0.60 or less, more preferably 0.58 orless. Reducing the circularity increases the entanglement betweenparticles during compaction and improves the compressibility of themixed powder for powder metallurgy, so that coarse pores in the greencompact and the sintered body are reduced. On the other hand, anexcessively low circularity reduces the compressibility of the mixedpowder for powder metallurgy. Accordingly, the circularity is preferably0.40 or more.

The circularity of the partially alloyed steel powder particles having adiameter of 50 μm to 100 μm can be measured as follows. First, theparticle diameter of the partially alloyed steel powder particles iscalculated in the same manner as that of the above-described iron-basedpowder particles and is expressed as dc, and the partially alloyed steelpowder particles having dc in a range of 50 μm to 100 μm are extracted.Here, optical microscopy imaging performed is such that at least 150particles of the partially alloyed steel powder that have a diameter ina range of 50 μm to 100 μm can be extracted. The circularity of theextracted partially alloyed steel powder particles was calculated in thesame manner as in the case of the above-described iron-based powder.

Note that the particle diameter of the partially alloyed steel powderparticles is limited to 50 μm to 100 μm because reducing the circularityof the particles of this range can most effectively promote sintering.Specifically, since particles of less than 50 μm are fine particleswhich originally facilitate sintering, reducing the circularity of suchparticles of less than 50 μm does not significantly promote sintering.Further, since particles having a particle diameter exceeding 100 μm areextremely coarse, reducing the circularity of those particles does notsignificantly promote sintering.

In this disclosure, the remainder components in the partially alloyedsteel powder are iron and inevitable impurities. Here, impuritiescontained in the partially alloyed steel powder may be C (except forgraphite content), O, N, S, and others, the contents of which may be setto C: 0.02% or less, 0: 0.3% or less, N: 0.004% or less, S: 0.03% orless, Si: 0.2% or less, Mn: 0.5% or less, and P: 0.1% or less in thepartially alloyed steel powder without any particular problem. Thecontent of O, however, is preferably 0.25% or less. It should be notedthat when the amount of incidental impurities exceeds the above range,the compressibility in compaction using the partially alloyed steelpowder decreases, which makes it difficult to obtain a green compacthaving sufficient density by the compaction.

In this disclosure, a sintered body manufactured using mixed powder forpowder metallurgy is further subjected to carburizing, quenching, andtempering, and Cu powder and graphite powder are then added to thepartially alloyed steel powder obtained as described above for thepurpose of achieving a tensile strength of 1000 MPa.

(Cu Powder)

Cu is an element useful in improving the solid solution strengtheningand the hardenability of iron-based powder thereby increasing thestrength of sintered parts. The amount of Cu added is preferably 0.5% ormore and 4.0 or less. When the amount of Cu powder added is less than0.5%, the advantageous effects of adding Cu are hardly obtained. On theother hand, when the Cu content exceeds 4.0%, not only does the effectsimproving the strength of the sintered parts reach a plateau but alsothe density of the sintered body is reduced. Therefore, the amount of Cupowder added is limited to a range of 0.5% to 4.0%.The amount added ispreferably in a range of 1.0% to 3.0%.

Further, when Cu powder of large particle size is used, in sintering agreen compact of mixed powder for powder metallurgy, molten Cupenetrates between particles of the partially alloyed steel powder toexpand the volume of the sintered body after sintering, which wouldreduce the density of the sintered body. In order to prevent the densityof the sintered body from decreasing in such a way, the mean particlediameter of the Cu powder is preferably set to be 50 μm or less. Morepreferably, the mean particle diameter of the Cu powder is 40 μm orless, still more preferably 30 μm or less. Although the lower limit ofthe mean particle diameter of the Cu powder is not specified, the lowerlimit is preferably set to be approximately 0.5 μm in order not toincrease the production cost of the Cu powder unnecessarily.

The mean particle diameter of the Cu powder can be calculated by thefollowing method.

Since the mean particle diameter of particles having a mean particlediameter of 45 μm or less is difficult to be measured by means ofsieving, the particle diameter is measured using a laserdiffraction/scattering particle size distribution measurement system.Examples of the laser diffraction/scattering particle size distributionmeasurement system include LA-950V2 manufactured by HORIBA, Ltd. Ofcourse, other laser diffraction/scattering particle size distributionmeasurement systems may be used; however, for performing accuratemeasurement, the lower limit and the upper limit of the measurableparticle diameter range of the system used are preferably 0.1 μm or lessand 45 μm or more, respectively. Using the system mentioned above, asolvent in which Cu powder is dispersed is exposed to a laser beam, andthe particle size distribution and the mean particle diameter of the Cupowder are measured from the diffraction and scattering intensity of thelaser beam. For the solvent in which the Cu powder is dispersed, ethanolis preferably used, since particles are easily dispersed in ethanol, andethanol is easy to handle. When a solvent in which the Van der Waalsforce is strong and particles are hardly dispersed, such as water isused, particles agglomerate during the measurement, and the measurementresult includes a mean particle diameter larger than the real meanparticle diameter. Therefore, such a solvent is not preferred.Accordingly, it is preferable that Cu powder introduced into an ethanolsolution is preferably dispersed using ultrasound before themeasurement.

Since the appropriate dispersion time varies depending on the targetpowder, the dispersion is performed in 7 stages at 10 min intervalsbetween 0 min and 60 min, and the mean particle diameter of the Cupowder is measured after each dispersion time stage. In order to preventparticle agglomeration, during each measurement, the measurement isperformed with the solvent being stirred. Of the particle diametersobtained through the seven measurements performed by changing thedispersion time by 10 min, the smallest value is used as the meanparticle diameter of the Cu powder.

(Graphite Powder)

Graphite powder is useful in increasing strength and fatigue strength,and graphite powder is added to the partially alloyed steel powder in anamount in a range of 0.1% to 1.0%, and mixing is performed. When theamount of graphite powder added is less than 0.1%, the aboveadvantageous effects cannot be obtained. On the other hand, when theamount of graphite powder added exceeds 1.0%, the sintered body becomeshypereutectoid, and cementite is precipitated, resulting in reducedstrength. Therefore, the amount of graphite powder added is limited to arange of 0.1% to 1.0%. The amount of graphite powder added is preferablyin a range of 0.2% to 0.8%. Note that the particle diameter of graphitepowder to be added is preferably in a range of approximately from 1 μmto 50 μm.

In this disclosure, the Cu powder and graphite powder described aboveare mixed with partially diffusion alloyed steel powder to which Mo isdiffusionally adhered to obtain Fe-Mo-Cu-C-based mixed powder for powdermetallurgy, and the mixing may be performed in accordance withconventional powder mixing methods.

Further, in a stage where a sintered body is obtained, if the sinteredbody needs to be further formed into the shape of parts by cutting workor the like, powder for improving machinability, such as MnS is added tothe mixed powder for powder metallurgy in accordance with conventionalmethods.

Next, the compacting conditions and sintering conditions preferable formanufacturing a sintered body using the-above described mixed powder forpowder metallurgy will be described.

In compaction using the above mixed powder for powder metallurgy, alubricant powder may also be mixed in. Further, compaction may beperformed with a lubricant being applied or adhered to a mold. In eithercase, as the lubricant, any of metal soap such as zinc stearate andlithium stearate, amide-based wax such as ethylenebisstearamide, andother well known lubricants may suitably be used. When mixing thelubricant, the amount thereof is preferably around from 0.1 parts bymass to 1.2 parts by mass with respect to 100 parts by mass of the mixedpowder for powder metallurgy.

In manufacturing a green compact by compacting the disclosed mixedpowder for powder metallurgy, the compaction is preferably performed ata pressure of 400 MPa to 1000 MPa. When the compacting pressure is lessthan 400 MPa, the density of the resulting green compact is reduced, andthe properties of the sintered body are degraded. On the other hand, acompacting pressure exceeding 1000 MPa extremely shortens the life ofthe mold, which is economically disadvantageous. The compactingtemperature is preferably in a range of room temperature (approximately20° C.) to approximately 160° C.

Further, the green compact is sintered preferably at a temperature in arange of 1100° C. to 1300° C. When the sintering temperature is lowerthan 1100° C., sintering stops; accordingly, it is difficult to achievethe desired tensile strength: 1000 MPa or more. On the other hand, asintering temperature higher than 1300° C. extremely shortens the lifeof a sintering furnace, which is economically disadvantageous. Thesintering time is preferably in a range of 10 min to 180 min.

A sintered body obtained using mixed powder for powder metallurgyaccording to this disclosure under the above sintering conditionsthrough such a procedure can have higher density after sintering thanthe case of using alloy steel powder which does not fall within theabove range even if the green density is the same.

Further, the resulting sintered body may be subjected to strengtheningprocesses such as carburized quenching, bright quenching, inductionhardening, and a carbonitriding process as necessary; however, even whensuch strengthening processes are not performed, the sintered body usingthe mixed powder for powder metallurgy according to this disclosure haveimproved strength and toughness compared with conventional sinteredbodies which are not subjected to strengthening processes. Thestrengthening processes may be performed in accordance with conventionalmethods.

The disclosed iron-based sintered body obtained as described abovepreferably contains Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5 mass % to 4.0mass %, and C: 0.1 mass % to 1.0 mass %. Specifically, the C content ispreferably in a range of 0.1% to 1.0% with which the higheststrengthening effect and the highest fatigue strength improving effectcan be achieved. When the C content is less than 0.1%, the aboveadvantageous effects cannot be achieved. On the other hand, a C contentexceeding 1.0% results in a hypereutectoid sintered body, so thatcementite is precipitated, resulting in reduced strength. Therefore, theamount of C contained in the sintered body is limited to a range of 0.1%to 1.0%. Preferably, the C content is 0.2% to 0.8%. The preferredcontent of Mo and Cu is determined as described above for the samereasons as in the case of the above-described mixed powder for powdermetallurgy.

Note that when a lubricant and the like are mixed into the above mixedpowder for powder metallurgy in manufacturing a sintered body, theamount of Mo, Cu, and C in the mixed powder for powder metallurgy iscontrolled so that the amount of Mo, Cu, and C contained the sinteredbody fall within the above range.

Further, the C content of the sintered body may change from the amountof graphite added depending on the sintering conditions (temperature,time, atmosphere, and others). Accordingly, when the amount of thegraphite powder added is controlled within the above range depending onthe sintering conditions, an iron-based sintered body having a C contentpreferred in this disclosure (0.1% to 1.0%, more preferably 0.2% to0.8%) can be manufactured.

EXAMPLES

A more detailed description of this disclosure will be given below withreference to examples; however, the disclosure is not limited solely tothe following examples.

Example 1

As-atomized powders having particles with different circularities wereused as iron-based powders. The as-atomized powders were subjected togrinding using a high speed mixer (LFS-GS-2J manufactured by FukaePowtec Corp.) so that the circularities of the particles varied.

Oxidized Mo powder (mean particle diameter: 10 μm) was added to theiron-based powders at a predetermined ratio, and the resultant powderswere mixed for 15 minutes in a V blender, then subjected to heattreatment in a hydrogen atmosphere with a dew point of 30° C. (holdingtemperature: 880° C., holding time: 1 h). Mo of a predetermined amountpresented in Table 1 was then adhered to the surface of the particles ofthe iron-based powders by diffusion bonding to produce partially alloyedsteel powders for powder metallurgy. Note that the Mo content was variedas in Samples Nos. 1 to 8 presented in Table 1.

The produced partially alloyed steel powders were each embedded into aresin and polishing was performed to expose a cross section of thepartially alloyed steel powder particles. Specifically, the partiallyalloyed steel powders were each embedded to be uniformly distributed inan area with a thickness of 0.5 mm or more in a thermosetting resin sothat a cross section of a sufficient number of partially alloyed steelpowder particles can be observed in the polished surface, that is, theobservation surface. After the polishing, the polished surface wasmagnified and imaged by an optical microscope, and the circularity ofthe particles was calculated by image analysis as described above.

Further, the specific surface area of the partially alloyed steel powderparticles was measured through BET theory. The particles of eachpartially alloyed steel powder were confirmed to have a specific surfacearea of less than 0.10 m²/g.

Subsequently, Cu powder of the mean particle diameter and the amountpresented in Table 1 was added to these partially alloyed steel powders,and graphite powder (mean particle diameter: 5 μm) of the amount alsopresented in Table 1 was added thereto. Ethylenebisstearamide was thenadded in an amount of 0.6 parts by mass to the resulting mixed powderfor powder metallurgy: 100 parts by mass, and the powder was then mixedin a V blender for 15 minutes.

Samples Nos. 9 to 25 used partially alloyed steel powder equivalent tothose used in Sample No. 5, yet the amounts of Cu powders and graphitepowders varied. Samples Nos. 26 to 31 used basically the same partiallyalloyed steel powder as that of Sample No. 5, of which mean particlediameter was adjusted by sieving. Further, Samples Nos. 32 to 38 usedpartially alloyed steel powders having circularities that varied.

After that, each mixed powder was compacted at a density of 7.0 g/cm³,thereby manufacturing bar-shaped green compacts having length: 55 mm,width: 10 mm, and thickness: 10 mm and ring-shaped green compacts havingouter diameter: 38 mm, inner diameter: 25 mm, and thickness: 10 mm (tenpieces each). The compacting pressure was 400 MPa in each case.

The bar-shaped green compacts and the ring-shaped green compacts weresintered thereby obtaining sintered bodies. The sintering was performedunder a set of conditions including sintered temperature: 1130° C. andsintering time: 20 min in a propane converted gas atmosphere.

The measurement of outer diameter, inner diameter, and thickness andmass measurement were performed on the ring-shaped sintered bodies,thereby calculating the sintered body density (Mg/m³). Further, themedian size, area fraction, and mean maximum pore length of pores in thesintered bodies were measured in accordance with the above-describedmethods.

For the bar-shaped sintered bodies, five of them were worked into roundbar tensile test pieces (JIS No. 2), each having a parallel portion witha diameter of 5 mm, to be subjected to the tensile test according to JISZ2241, and the other five were bar shaped (unnotched) as sintered andhad a size according to JIS Z2242 to be subjected to the Charpy impacttest according to JIS Z2242. Each of these test pieces was subjected togas carburizing at carbon potential: 0.8 mass % (holding temperature:870° C., holding time: 60 min) followed by quenching (60° C., oilquenching) and tempering (holding temperature: 180° C., holding time: 60min).

The round bar tensile test pieces and bar-shaped test pieces for theCharpy impact test subjected to carburizing, quenching, and temperingwere subjected to the tensile test according to JIS Z2241 and the Charpyimpact test according to JIS Z2242; thus, the tensile strength (MPa) andthe impact energy value (J/cm²) were measured and the mean values werecalculated with the number of samples n=5.

The measurement results are also presented in Table 1. The evaluationcriteria are as follows.

-   (1) Flowability of Mixed Powder

Mixed powders for powder metallurgy: 100 g were introduced into a nozzlehaving diameter: 2.5 mmφ. When the total amount of powder was completelyflown within 80 s without stopping, the powder was judged to have passed(passed). When the powder required a longer time to be flown or thetotal amount or part of the amount of powder stopped and failed to beflown, the powder was judged to have failed (failed).

-   (2) Sintered Body Density

A sintered body density of 6.95 Mg/m³ or more, that is equal to orhigher than that of a conventional 4Ni material (4 Ni-1.5 Cu-0.5 Mo,maximum particle diameter of material powder: 180 μm) was judged to havepassed.

-   (3) Tensile Strength

When the round bar tensile test pieces having been subjected tocarburizing, quenching, and tempering had a tensile strength of 1000 MPaor more, the test pieces were judged to have passed.

-   (4) Impact Energy Value

When the bar-shaped test pieces for the Charpy impact test having beensubjected to carburizing, quenching, and tempering had an impact energyvalue of 14.5 J/cm² or more, the test pieces were judged to have passed.

Note that the test of the impact energy value was also performed on thesintered body before carburizing, quenching, and tempering.

TABLE 1 After carburizing, Partially alloyed quenching, steel powderSintered body tempering Mean Mo Cu Graphite Cu Mo Cu C Pore Mean ImpactImpact particle content content content particle content content contentarea Median maximum energy Tensile energy Sample diameter (mass (mass(mass diameter (mass (mass (mass fraction pore size pore size valueDensity strength value No. (μm) Circularity %) %) %) (μm) Flowability %)%) %) (%) (μm) (μm) (J/cm²) (Mg/m³) (MPa) (J/cm²) Evaluation Note  1 890.58 0.1 2.0 0.30 35 passed 0.1 2.0 0.3 14 22 110 24 7.02 1080 13.8failed Comparative Example  2 91 0.60 0.2 2.0 0.30 35 passed 0.2 2.0 0.314 16 90 33 7.00 1125 14.7 passed Example  3 92 0.61 0.4 2.0 0.30 35passed 0.4 2.0 0.3 14 14 80 41 7.01 1150 15.6 passed Example  4 95 0.620.6 2.0 0.30 35 passed 0.6 2.0 0.3 14 15 82 40 7.01 1175 15.4 passedExample  5 91 0.58 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 15 16 80 39 6.971185 15.1 passed Example  6 88 0.63 1.0 2.0 0.30 35 passed 1.0 2.0 0.314 17 87 36 6.98 1195 14.8 passed Example  7 92 0.63 1.5 2.0 0.30 35passed 1.5 2.0 0.3 15 17 90 34 6.95 1200 14.6 passed Example  8 93 0.622.0 2.0 0.30 35 passed 2.0 2.0 0.3 15 22 110 22 6.93 1230 13.6 failedComparative Example  9 91 0.58 0.8 0.2 0.30 35 passed 0.8 0.2 0.3 14 2192 23 7.01 980 13.6 failed Comparative Example 10 91 0.58 0.8 0.5 0.3035 passed 0.8 0.5 0.3 14 19 86 32 7.00 1015 14.6 passed Example 11 910.58 0.8 1.5 0.30 35 passed 0.8 1.5 0.3 14 15 84 37 6.98 1135 15.1passed Example 12 91 0.58 0.8 3.0 0.30 35 passed 0.8 3.0 0.3 15 14 78 396.97 1210 15.4 passed Example 13 91 0.58 0.8 4.0 0.30 35 passed 0.8 4.00.3 15 14 75 42 6.95 1180 15.9 passed Example 14 91 0.58 0.8 5.0 0.30 35passed 0.8 5.0 0.3 16 22 70 23 6.92 990 13.0 failed Comparative Example15 91 0.58 0.8 2.0 0.05 35 passed 0.8 2.0 0.05 14 13 73 41 7.02 980 16.0failed Example 16 91 0.58 0.8 2.0 0.15 35 passed 0.8 2.0 0.2 14 17 85 387.00 1090 15.2 passed Example 17 91 0.58 0.8 2.0 0.50 35 passed 0.8 2.00.5 14 16 90 32 6.98 1150 14.8 passed Example 18 91 0.58 0.8 2.0 1.00 35passed 0.8 2.0 1.0 15 20 105 28 6.97 1180 14.5 passed Example 19 91 0.580.8 2.0 1.50 35 passed 0.8 2.0 1.5 15 26 125 20 6.97 1115 12.0 failedComparative Example 20 91 0.58 0.8 2.0 0.30 55 passed 0.8 2.0 0.3 15 1984 30 6.95 1110 14.5 passed Example 21 91 0.58 0.8 2.0 0.30 48 passed0.8 2.0 0.3 15 18 87 29 6.96 1140 14.6 passed Example 22 91 0.58 0.8 2.00.30 30 passed 0.8 2.0 0.3 14 15 83 35 6.98 1151 15.1 passed Example 2391 0.58 0.8 2.0 0.30 24 passed 0.8 2.0 15 82 36 6.99 1160 15.1 passedExample 24 91 0.58 0.8 2.0 0.30 15 passed 0.8 2.0 0.3 14 16 81 38 7.001180 15.2 passed Example 25 91 0.58 0.8 2.0 0.30 1.5 passed 0.8 2.0 0.313 14 76 39 7.03 1210 15.6 passed Example 26 128 0.48 0.8 2.0 0.30 35passed 0.8 2.0 0.3 16 22 100 29 6.93 995 14.0 failed Comparative Example27 118 0.55 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 18 82 31 6.98 115014.7 passed Example 28 98 0.57 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 1678 37 7.00 1135 15.4 passed Example 29 75 0.58 0.8 2.0 0.30 35 passed0.8 2.0 0.3 14 15 74 42 7.01 1194 15.7 passed Example 30 60 0.59 0.8 2.00.30 35 passed 0.8 2.0 0.3 14 14 73 44 7.01 1230 16.0 passed Example 3135 0.62 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 13 72 46 6.99 1260 16.3passed Example 32 28 0.64 0.8 2.0 0.30 35 failed — — — — — — — — — —failed Comparative Example 33 70 0.45 0.8 2.0 0.30 35 passed 0.8 2.0 0.314 12 71 47 7.01 1240 16.4 passed Example 34 69 0.54 0.8 2.0 0.30 35passed 0.8 2.0 0.3 14 14 70 45 7.00 1213 16.1 passed Example 35 72 0.560.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 14 74 43 6.99 1180 15.9 passedExample 36 69 0.60 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 17 84 38 7.001140 15.0 passed Example 37 70 0.62 0.8 2.0 0.30 35 passed 0.8 2.0 0.315 19 85 35 6.97 1120 14.7 passed Example 38 71 0.67 0.8 2.0 0.30 35passed 0.8 2.0 0.3 14 28 100 19 6.98 1001 12.0 failed ComparativeExample  39* 65 0.67 0.5 — 0.30 35 passed 0.5 1.5 0.3 15 29 130 25 6.97998 13.3 failed Comparative Example Sample No. 39 is a 4Ni material(Fe—4Ni—1.5Cu—0.5Mo) For each of Samples Nos. 1, 8, 9, 14, 19, 26, 38,and 39*, the median size D50 of the pores in the sintered body exceeded20 μm, resulting in a low impact energy value, lack of toughness, andreduced tensile strength.

Further, for comparing the effects of the components in the sinteredbodies, the Mo content in Sample Nos. 1 to 8, the Cu content in Nos. 9to 14, and the graphite content in Nos. 15 to 19 were contrasted.Similarly, Samples Nos. 20 to 25 were designed for evaluating the effectof the Cu particle diameter, Nos. 26 to 31 for evaluating the effect ofthe alloyed particle diameter, and Nos. 32 to 38 for evaluating theeffect of the circularity and the mean particle diameter of thepartially alloyed steel powders. Table 1 also presents the results of a4 Ni material (4 Ni-1.5 Cu-0.5 Mo, maximum particle diameter of materialpowder: 180 μm) as the conventional material. The table demonstratesthat our examples exhibited better properties over the conventional 4 Nimaterial.

As presented in Table 1, all of Examples of this disclosure weresintered bodies having high tensile strength and toughness.

Example 2

Three atomized iron powders having particles of different specificsurface areas and circularities were prepared. The specific surface areaand the circularity were adjusted by grinding each atomized iron powderusing a high speed mixer (LFS-GS-2J manufactured by Fukae Powtec Corp.)and adjusting the mixing ratio of coarse powder having a particle sizeof 100 μm or more and fine powder having a particle size of 45 μm orless.

Oxidized Mo powder (mean particle diameter: 10 μm) was added to theiron-based powders at a predetermined ratio, and the resultant powderswere mixed for 15 minutes in a V blender, then subjected to heattreatment in a hydrogen atmosphere with a dew point of 30° C. (holdingtemperature: 880° C., holding time: 1 h). Mo of a predetermined amountpresented in Table 2 was then adhered to the surface of the particles ofthe iron-based powders by diffusion bonding to produce partially alloyedsteel powders for powder metallurgy. These partially alloyed steelpowders were each embedded into a resin and polishing was performed toexpose a cross section of the partially alloyed steel powder particles.Subsequently, the cross section was magnified and imaged by an opticalmicroscope, and the circularity of the particles was calculated by imageanalysis. Further, the specific surface area of the partially alloyedsteel powder particles was measured through BET theory.

Next, 2 mass % of Cu powder having a mean particle diameter of 35 μm wasadded to these partially alloyed steel powders, and 0.3 mass % ofgraphite powder (mean particle diameter: 5 μm) was added thereto.Ethylenebisstearamide was then added in an amount of 0.6 parts by massto the resulting mixed powder for powder metallurgy: 100 parts by mass,and the powder was then mixed in a V blender for 15 minutes. Each of themixed powders was compacted at a compacting pressure of 686 MPa, therebymanufacturing bar-shaped green compacts having length: 55 mm, width: 10mm, and thickness: 10 mm and ring-shaped green compacts having outerdiameter: 38 mm, inner diameter: 25 mm, and thickness: 10 mm (ten pieceseach).

The bar-shaped green compacts and ring-shape green compacts weresintered to obtain sintered bodies. The sintering was performed under aset of conditions including sintered temperature: 1130° C. and sinteringtime: 20 min in a propane converted gas atmosphere.

The measurement of outer diameter, inner diameter, and thickness andmass measurement were performed on the ring-shaped sintered bodies,thereby calculating the sintered body density (Mg/m³). Further, themedian size, area fraction, and mean maximum pore length of pores in thesintered bodies were measured in accordance with the above-describedmethods.

For the bar-shaped sintered bodies, five of them were worked into roundbar tensile test pieces (JIS No. 2) having diameter: 5 mm to besubjected to the tensile test according to JIS Z2241, and the other fivewere bar shaped (unnotched) as sintered to be subjected to the Charpyimpact test according to JIS Z2242. Each of these test pieces wassubjected to gas carburizing at carbon potential: 0.8 mass % (holdingtemperature: 870° C., holding time: 60 min) followed by quenching (60°C., oil quenching) and tempering (holding temperature: 180° C., holdingtime: 60 min).

The round bar tensile test pieces and bar-shaped test pieces for theCharpy impact test subjected to carburizing, quenching, and temperingwere subjected to the tensile test according to JIS Z2241 and the Charpyimpact test according to JIS Z2242; thus, the tensile strength (MPa) andthe impact energy value (J/cm²) were measured and the mean values werecalculated with the number of samples n=5.

The measurement results are also presented in Table 2. The acceptancecriteria for the values of the properties were the same as those inExample 1.

TABLE 2 Partially alloyed steel powder Mean Specific Cu Sintered bodyparticle surface Mo Cu Graphite particle Mo Cu Sample diameter areacontent content content diameter content content No. (μm) Circularity(m²/g) (mass %) (mass %) (mass %) (μm) Flowability (mass %) (mass %) 4078 0.55 0.07 0.4 2.0 0.3 35 passed 0.4 2.0 41 76 0.52 0.08 0.8 2.0 0.335 passed 0.8 2.0 42 76 0.59 0.13 0.4 2.0 0.3 35 — 0.4 2.0 43 77 0.520.15 0.8 2.0 0.3 35 — 0.8 2.0 44 76 0.67 0.12 0.4 2.0 0.3 35 — 0.4 2.045 77 0.66 0.14 0.8 2.0 0.3 35 — — — 46 75 0.68 0.06 0.4 2.0 0.3 35passed 0.4 2.0 47 77 0.69 0.08 0.8 2.0 0.3 35 passed 0.8 2.0 Aftercarburizing, quenching, Sintered body tempering Mean Impact Impact CPore area Median maximum energy Tensile energy Sample content fractionpore size pore size value Density strength value No. (mass %) (%) (μm)(μm) (J/cm²) (Mg/m³) (MPa) (J/cm²) Evaluation Note 40 0.3 14 16 85.035.0 7.01 1175 15.1 passed Example 41 0.3 15 14 73.0 42.0 6.97 1194 15.7passed Example 42 0.3 — — — — — — — failed Comparative Example 43 0.3 —— — — — — — failed Comparative Example 44 0.3 — — — — — — — failedComparative Example 45 — — — — — — — — failed Comparative Example 46 0.312 25 110.0 21.0 7.10 1060 12.1 failed Comparative Example 47 0.3 13 25100.0 20.0 7.06 1075 12.3 failed Comparative Example

As can be seen from Table 2, all the sintered bodies having a medianpore size D50 of 20 μm or less had a high impact energy value, excellenttoughness, and high tensile strength. Further, when partially alloyedsteel powders having particles of a circularity and a specific surfacearea within the disclosed range were used, the target values of thesintered body density, the tensile strength, and the impact energy valuewere achieved.

1.-9. (canceled)
 10. An iron-based sintered body, comprising an areafraction of pores in the iron-based sintered body of 15% or less, and anarea-based median size D50 of the pores of 20 μm or less.
 11. Theiron-based sintered body according to claim 10, comprising Mo, Cu, andC.
 12. The iron-based sintered body according to claim 11, comprising Moin an amount of 0.2 mass % to 1.5 mass %, Cu in an amount of 0.5 mass %to 4.0 mass %, and C in an amount of 0.1 mass % to 1.0 mass %.
 13. Theiron-based sintered body according to claim 10, wherein the iron-basedsintered body has been carburized, quenched, and tempered.
 14. Theiron-based sintered body according to claim 11, wherein the iron-basedsintered body has been carburized, quenched, and tempered.
 15. Theiron-based sintered body according to claim 12, wherein the iron-basedsintered body has been carburized, quenched, and tempered.
 16. A methodof manufacturing an iron-based sintered body, the method comprising:compacting (i) partially diffusion alloyed steel powder in which Mo isadhered to the surface of particles of iron-based powder by diffusionbonding with (ii) mixed powder for powder metallurgy obtained by mixingat least Cu powder and graphite powder at a pressure of 400 MPa or moreto obtain a compact; and then sintering the obtained compact at 1000° C.or higher for 10 min or more.
 17. The method of manufacturing a highstrength according to claim 16, the method further comprisingcarburizing, quenching, and tempering after sintering the obtainedcompact.
 18. The method of manufacturing an iron-based sintered body,according to claim 16, wherein the mixed powder for powder metallurgycontains Mo in an amount of 0.2 mass % to 1.5 mass % and the balanceconsisting of Fe and incidental impurities.
 19. The method ofmanufacturing an iron-based sintered body, according to claim 17,wherein the mixed powder for powder metallurgy contains Mo in an amountof 0.2 mass % to 1.5 mass % and the balance consisting of Fe andincidental impurities.
 20. The method of manufacturing an iron-basedsintered body, according to claim 16, wherein the partially diffusionalloyed steel powder has a mean particle diameter of 30 μm to 120 μm anda specific surface area of less than 0.10 m²/g, and a circularity ofparticles of the partially diffusion alloyed steel powder that have adiameter in a range of 50 μm to 100 μm is 0.65 or less.
 21. The methodof manufacturing an iron-based sintered body, according to claim 17,wherein the partially diffusion alloyed steel powder has a mean particlediameter of 30 μm to 120 μm and a specific surface area of less than0.10 m²/g, and a circularity of particles of the partially diffusionalloyed steel powder that have a diameter in a range of 50 μm to 100 μmis 0.65 or less.
 22. The method of manufacturing an iron-based sinteredbody, according to claim 18, wherein the partially diffusion alloyedsteel powder has a mean particle diameter of 30 μm to 120 μm and aspecific surface area of less than 0.10 m²/g, and a circularity ofparticles of the partially diffusion alloyed steel powder that have adiameter in a range of 50 μm to 100 μm is 0.65 or less.
 23. The methodof manufacturing an iron-based sintered body, according to claim 19,wherein the partially diffusion alloyed steel powder has a mean particlediameter of 30 μm to 120 μm and a specific surface area of less than0.10 m²/g, and a circularity of particles of the partially diffusionalloyed steel powder that have a diameter in a range of 50 μm to 100 μmis 0.65 or less.
 24. The method of manufacturing an iron-based sinteredbody, according to claim 16, wherein the amount of the Cu powder mixedis 0.5 mass % to 4.0 mass % of the mixed powder for powder metallurgy.25. The method of manufacturing an iron-based sintered body, accordingto claim 17, wherein the amount of the Cu powder mixed is 0.5 mass % to4.0 mass % of the mixed powder for powder metallurgy.
 26. The method ofmanufacturing an iron-based sintered body, according to claim 18,wherein the amount of the Cu powder mixed is 0.5 mass % to 4.0 mass % ofthe mixed powder for powder metallurgy.
 27. The method of manufacturingan iron-based sintered body, according to claim 19, wherein the amountof the Cu powder mixed is 0.5 mass % to 4.0 mass % of the mixed powderfor powder metallurgy.
 28. The method of manufacturing an iron-basedsintered body, according to claim 20, wherein the amount of the Cupowder mixed is 0.5 mass % to 4.0 mass % of the mixed powder for powdermetallurgy.
 29. The method of manufacturing an iron-based sintered body,according to claim 21, wherein the amount of the Cu powder mixed is 0.5mass % to 4.0 mass % of the mixed powder for powder metallurgy.